1. Technical Field
This invention relates generally to data communication between stages of photographic filmstrip use and processing wherein data is recorded in a magnetic layer of the photographic filmstrip at a first stage and read out at a second stage. More particularly, the invention relates to a method and apparatus for reducing overhead and data loss due to drop-outs and other data corruption errors, in such data transfer.
2. Background Information
Data communication between different stages of film use and processing (e.g. a camera user and dealer or photofinisher) has traditionally required separate written forms. This has not proven to be a very convenient or efficient method of relaying important information from one stage to another.
In the early 1960's, an innovation in data communication for motion picture film was introduced. A thin layer of magnetic oxide, referred to as a DATAKODE Magnetic Control Surface, was coated across the entire back surface of a roll of motion picture film to provide the capability to magnetically record digital data on the film without interfering with normal photographic use of the film.
The DATAKODE Magnetic Control Surface permitted recording of different types of digital data at different stages of production of a motion picture. Such data could range from camera, lighting and filter data at the time of shooting to printer exposure control information in the laboratory to theatre automation control signals during exhibition. The availability of the DATAKODE Magnetic Control Surface over the entire surface of the motion picture film allowed multiple types of data to be recorded on the same piece of film.
The DATAKODE Magnetic Control Surface was specifically applied to the recording of SMPTE time code on motion picture films. Two formats were proposed for recording the SMPTE time code: a standard continuous longitudinal time code format, and a frame limited burst type format. The latter format was preferred because of its ready adaption to the intermittent motion of the film during normal picture exposure and projection operations.
The SMPTE time code served essentially as a machine-readable frame address code for the motion picture film. The continuous time code included 26 bits of frame identification, 32 spare user bits, a 16-bit sync word, and 6 other miscellaneous data bits. The burst type code added a 16-bit sync word and an 8-bit preamble to the beginning and an 8-bit postamble to the end of the 80 bits of continuous time code. Gaps between time code bursts of different frames were filled with a pattern of alternating ones and zeros.
Regardless of whether the continuous or burst format was employed, the binary structure of each data field was exactly the same as every other data field, pre-defined by the SMPTE time code application.
More recently, a film information exchange system using dedicated magnetic tracks has been developed for use in still photography. A virtually transparent magnetic layer on the still photography filmstrip facilitates the magnetic recording of data in one or more longitudinal tracks of each film frame. With a virtually transparent magnetic layer, data recording may be done everywhere on the film including in the image area, so that all relevant information can be theoretically recorded with each frame of the film. In order to provide quick access to particular data at any stage of film use, related data is preferably grouped and recorded in specific predetermined tracks. Camera data, for example, can be recorded in several dedicated longitudinal tracks along the filmstrip edges. The camera data, as well as other data, is preferably recorded in pulse position encoded form in order to be independent of film transport velocity.
In this earlier system, each track is preferably frame limited and a two character start sentinel is recorded at the beginning of the track and a two character end sentinel is recorded at the end of the track. Between the start sentinel and end sentinel, a plurality of consecutive self-identifying data fields can be recorded. Each field begins with a single character field sentinel followed by a two character identification (ID) code, appropriate data characters and a parity character. The field sentinel signifies the beginning of each field and preferably identifies the source of the data in the field. The identification code, by reference to a dictionary stored in memory, identifies the nature of the data in the field and an appropriate decoding scheme. The parity character facilitates error correction for the ID code and data characters within the field. This basic data architecture, as well an alternative employing virtual identification codes, are fully described in commonly assigned U.S. Pat. No. 4,965,627, the disclosure of which is incorporated by reference herein.
The self-identifying data field feature of the Film Information Exchange System Using Dedicated Magnetic Tracks facilitates rapid accessing and reading of particular desired data by different stages of film use and processing. These and other significant advantages and benefits are fully described in U.S. Pat. No. 4,965,627 and the related patents referenced therein.
However, under certain circumstances, the data architecture of this earlier system may not be optimal. If a drop-out (e.g. lost bit) occurs early in a track, decoding of all subsequent data in the track may be compromised, potentially resulting in significant data loss or difficult data reconstruction. Also, particularly when a short data field is to be repetitively recorded, the Film Information Exchange System Using Dedicated Magnetic Tracks may involve excess overhead. Overhead refers to characters or bits recorded for control or identification purposes. Commonly assigned, concurrently filed U.S. patent application Ser. No. 07/811390 entitled METHOD AND APPARATUS FOR MAGNETICALLY COMMUNICATING VIA A PHOTOGRAPHIC FILMSTRIP WITH ENHANCED RELIABILITY by Arthur Whitfield, et al., describes an approach for extending the Film Information Exchange System to a simple, low cost camera, in which a data field containing a limited data set is repetitively recorded along a track of a film frame. In such systems, it is desirable to reduce overhead in order to increase the number of repeating data fields that can be recorded within the limited track length.
In other environments, various approaches have been developed for formatting data to be recorded in a magnetic track. U.S. Pat. No. 4,835,628 to Hinz, et al. describes an apparatus and method for formatting and recording digital data in discrete stripes on magnetic tape using a helical scan arrangement. Formatting in the data area of each stripe includes recording of digital information within preamble, data block, and postamble sections. The preamble section provides frequency/phase and location referencing, the data block section includes a plurality of physical data blocks each of which are divided into sub-blocks that include synchronizing and segment identifying information along with data to be recorded, and the postamble section ensures compatibility of physical alignment between the recording heads and magnetic tape.
U.S. Pat. No. 4,422,111 to Moeller, et al. describes a method of preformatting magnetic tape intended for use in high capacity data cartridges by prerecording identifying blocks on the tape across the full width of the tape in order to segment the tape into identifying sections between which data can be recorded in eight tracks following a serpentine pattern. Data frames in the tracks comprise a preamble, header, data portion, CRC code word, and an interframe gap. The header portion includes a track and frame number, record number, a record type number, and a character count. By inference, it would appear that prior to looking for an address in this data structure one has to consult a dictionary and determine the tape address of the data of interest.
In addition to overhead and data loss considerations, the application of magnetics on photographic film to transfer data raises other unique concerns. As a magnetic recording material, photographic film is relatively thick and not as compliant as typical magnetic tape. Further, the recording environment in many cameras is not as controlled and can suffer from higher error rates due to unsophisticated film transports systems, low power batteries, poor head-to-film interface and other physical disturbances.
A need therefore exists for a data formatting approach which ensures the reliable transfer of data via magnetic recording on a photographic filmstrip with low overhead and data loss even when the data transfer is subject to such corruption errors.